1. Field of the Invention
The present invention relates to a room temperature curable organopolysiloxane composition, which yields a cured product that exhibits excellent adhesion when exposed to hot water vapor and following such exposure, exhibits minimal impurities (evaporation residues) within a solution obtained by extracting the cured product with water, and can be used as a sealing material for construction purposes or as an adhesive for bonding or securing components for electrical or electronic products.
2. Description of the Prior Art
RTV (room temperature vulcanizing) silicone rubber compositions that undergo cross-linking in the presence of moisture are not only easy to handle, but also exhibit excellent heat resistance, adhesion, and electrical characteristics, and are consequently used in a wide variety of fields as sealing materials for construction purposes and as adhesives within the electrical and electronic fields. In the case of sealing applications for components for electrical or electronic products, and particularly for the sealing of window frames in microwave ovens, silicone rubber compositions that undergo cross-linking in the presence of moisture are widely used because of their superior heat resistance and adhesion. In this application, the silicone rubber obtained by curing the composition must not only exhibit excellent initial adhesion to the glass and coated steel plate that function as adhesion targets, together with excellent heat resistant adhesion and excellent adhesion when exposed to hot water vapor, but also need to be safe from a food hygiene viewpoint. For example, in Japan, such a silicone rubber must pass a Japanese Food Hygiene Law test (Official Notification No. 85, Ministry of Health, Labor, and Welfare of Japan).
In order to impart adhesiveness to a RTV silicone rubber, the addition of a silicon compound-based silane coupling agent is effective, but although RTV silicone rubbers comprising conventionally used silane coupling agents such as aminopropyltriethoxysilane exhibit excellent levels of initial adhesion and heat resistant adhesion, they have been ineffective in improving the adhesion upon exposure to hot water vapor. For example, the patent reference 1 and patent reference 2 propose RTV silicone rubber compositions that use the reaction product of a mercaptosilane and an isocyanate compound or polyisocyanate compound as an adhesion assistant, and although the silicone rubbers obtained from these compositions exhibit excellent initial adhesion, the adhesion upon exposure to hot water vapor is inadequate.
Furthermore, the patent reference 3 proposes the use of a poly(alkoxysilylalkyl)amine as the cross-linking agent for a RTV silicone rubber composition, but the adhesion upon exposure to hot water vapor and the degree of reduction in the level of evaporation residues are inadequate.
In order to resolve the problems described above, the inventors of the present invention have already proposed a RTV silicone rubber composition that comprises a specific silane coupling agent (see patent reference 4). Compared with conventional RTV silicone rubber compositions, this RTV silicone rubber composition provides a dramatic improvement in the adhesion following exposure to hot water vapor. However, in the elution tests prescribed in the Official Notification No. 85 from the Ministry of Health, Labor, and Welfare of Japan, and particularly in the test relating to evaporation residues, the composition narrowly failed to reach the specified criteria.
[Patent Reference 1] U.S. Pat. No. 4,629,775
[Patent Reference 2] JP 61-64753A
[Patent Reference 3] EP 0 206 301 A2
[Patent Reference 4] US 2002/0091220 A1